AP enzyme uses a substrate that includes a phosphate group; identify this substrate from the options.

• A. Naphthol-AS-phosphate
• B. Hydrogen peroxide
• C. 3,3'-diaminobenzidine
• D. Tetramethylbenzidine

Answer: (A)

Alkaline phosphatase works on substrates that contain a phosphate group. It cleaves the phosphate to release a phenolic compound, which then combines with a diazonium salt to form a colored product. Among the options, the one with a phosphate group is the correct substrate for this enzyme, because it starts as a phosphate ester that AP can dephosphorylate.

Hydrogen peroxide is the oxidizing substrate that drives the color reaction for peroxidase enzymes, not alkaline phosphatase. The color-generating dyes 3,3'-diaminobenzidine and tetramethylbenzidine are chromogens used with peroxidase in the presence of hydrogen peroxide, not substrates for alkaline phosphatase.